diff --git a/app/services/incoming_emails.rb b/app/services/incoming_emails.rb
new file mode 100644
index 00000000000..b99355d4066
--- /dev/null
+++ b/app/services/incoming_emails.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module IncomingEmails
+  class UnauthorizedAction < StandardError; end
+  class MissingInformation < StandardError; end
+end
diff --git a/app/services/incoming_emails/dispatch_service.rb b/app/services/incoming_emails/dispatch_service.rb
index 61df87c1751..d550192c1e3 100644
--- a/app/services/incoming_emails/dispatch_service.rb
+++ b/app/services/incoming_emails/dispatch_service.rb
@@ -28,10 +28,6 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 module IncomingEmails
-  class UnauthorizedAction < StandardError; end
-
-  class MissingInformation < StandardError; end
-
   class DispatchService
     include ActionView::Helpers::SanitizeHelper
 
@@ -54,6 +50,10 @@ module IncomingEmails
       handlers.unshift(handler_class)
     end
 
+    def self.remove_handler(handler_class)
+      handlers.delete(handler_class)
+    end
+
     attr_reader :email, :sender_email, :user, :options, :success, :logs
 
     def initialize(email, options:)
@@ -94,6 +94,7 @@ module IncomingEmails
     # the subject. Additionally, the subject structure might change, e.g. via localization changes.
     def dispatch
       call = call_matching_handler
+      return if call.nil?
 
       @success = call.success?
       log_handler_call(call)
@@ -101,13 +102,10 @@ module IncomingEmails
       call.result
     rescue ActiveRecord::RecordInvalid => e
       log "could not save record: #{e.message}", :error
-      nil
     rescue MissingInformation => e
       log "missing information from #{user}: #{e.message}", :error
-      nil
     rescue UnauthorizedAction
       log "unauthorized attempt from #{user}", :error
-      nil
     end
 
     def log_handler_call(call)
@@ -127,7 +125,7 @@ module IncomingEmails
             :info,
             report: false
 
-        false
+        nil
       end
     end
 
@@ -262,10 +260,10 @@ module IncomingEmails
 
       message = "MailHandler: #{message}"
       Rails.logger.public_send(level, message)
+      nil
     end
 
-    def assign_options(value)
-      # rubocop:disable Metrics/AbcSize
+    def assign_options(value) # rubocop:disable Metrics/AbcSize
       options = value.dup
 
       options[:issue] ||= {}
@@ -280,6 +278,7 @@ module IncomingEmails
       options[:allow_override] << :type unless options[:issue].has_key?(:type)
       # Priority overridable by default
       options[:allow_override] << :priority unless options[:issue].has_key?(:priority)
+
       options[:no_permission_check] = ActiveRecord::Type::Boolean.new.cast(options[:no_permission_check])
 
       options
diff --git a/app/services/incoming_emails/handlers/base.rb b/app/services/incoming_emails/handlers/base.rb
index 35d51648d26..661ab141257 100644
--- a/app/services/incoming_emails/handlers/base.rb
+++ b/app/services/incoming_emails/handlers/base.rb
@@ -49,6 +49,10 @@ module IncomingEmails::Handlers
       raise NotImplementedError, "Subclasses must implement handle method"
     end
 
+    def cleaned_up_text_body
+      cleanup_body(plain_text_body)
+    end
+
     protected
 
     # The receive_* methods have been moved to specific handler classes:
@@ -188,7 +192,7 @@ module IncomingEmails::Handlers
       # * parse the email To field
       # * specific project (eg. Setting.mail_handler_target_project)
       target = Project.find_by(identifier: get_keyword(:project))
-      raise MissingInformation.new("Unable to determine target project") if target.nil?
+      raise IncomingEmails::MissingInformation.new("Unable to determine target project") if target.nil?
 
       target
     end
@@ -209,10 +213,6 @@ module IncomingEmails::Handlers
       end
     end
 
-    def cleaned_up_text_body
-      cleanup_body(plain_text_body)
-    end
-
     # Removes the email body of text after the truncation configurations.
     def cleanup_body(body)
       delimiters = Setting.mail_handler_body_delimiters.to_s.split(/[\r\n]+/).compact_blank.map { |s| Regexp.escape(s) }
diff --git a/app/services/incoming_emails/handlers/message_reply.rb b/app/services/incoming_emails/handlers/message_reply.rb
index c5fe2e831cb..d2b839eda37 100644
--- a/app/services/incoming_emails/handlers/message_reply.rb
+++ b/app/services/incoming_emails/handlers/message_reply.rb
@@ -42,7 +42,7 @@ module IncomingEmails::Handlers
     private
 
     # Receives a reply to a forum message
-    def receive_message_reply(message_id) # rubocop:disable Metrics/AbcSize
+    def receive_message_reply(message_id)
       message = Message.find_by(id: message_id)
       if message
         message = message.root
@@ -55,17 +55,22 @@ module IncomingEmails::Handlers
           ServiceResult.failure(message: "ignoring reply from [#{sender_email}] to a locked topic",
                                 mesage_type: :warn)
         else
-          reply = Message.new(subject: email.subject.gsub(%r{^.*msg\d+\]}, "").strip,
-                              content: cleaned_up_text_body)
-          reply.author = user
-          reply.forum = message.forum
-          message.children << reply
-          add_attachments(reply)
-          reply
-          ServiceResult.success(result: reply,
-                                message: "Reply added to message ##{message.id}")
+          create_reply_message(message)
         end
       end
     end
+
+    def create_reply_message(root)
+      reply = Message.new(subject: email.subject.gsub(%r{^.*msg\d+\]}, "").strip,
+                          content: cleaned_up_text_body)
+      reply.author = user
+      reply.forum = root.forum
+      root.children << reply
+      add_attachments(reply)
+      reply
+
+      ServiceResult.success(result: reply,
+                            message: "Reply added to message ##{root.id}")
+    end
   end
 end
diff --git a/app/services/incoming_emails/handlers/work_package.rb b/app/services/incoming_emails/handlers/work_package.rb
index edc8fe3a771..afeef87433e 100644
--- a/app/services/incoming_emails/handlers/work_package.rb
+++ b/app/services/incoming_emails/handlers/work_package.rb
@@ -73,17 +73,18 @@ module IncomingEmails::Handlers
 
       call = create_work_package(project)
 
-      call.message = if call.success?
-        "work_package created by #{user}"
-      else
-        "work_package could not be created by #{user} due to ##{call.errors.full_messages}"
-      end
+      call.message =
+        if call.success?
+          "work_package created by #{user}"
+        else
+          "work_package could not be created by #{user} due to ##{call.errors.full_messages}"
+        end
 
       call
     end
 
     # Adds a note to an existing work package
-    def receive_work_package_reply(work_package_id)
+    def receive_work_package_reply(work_package_id) # rubocop:disable Metrics/AbcSize
       work_package = ::WorkPackage.find_by(id: work_package_id)
       return unless work_package
 
diff --git a/app/services/incoming_emails/mail_handler.rb b/app/services/incoming_emails/mail_handler.rb
index ebdc15602f0..4bde340166d 100644
--- a/app/services/incoming_emails/mail_handler.rb
+++ b/app/services/incoming_emails/mail_handler.rb
@@ -33,7 +33,8 @@ module IncomingEmails
     ##
     # Code copied from base class and extended with optional options parameter
     # as well as force_encoding support.
-    def self.receive(raw_mail, options = {})
+    def self.receive(input, options = {})
+      raw_mail = input.dup
       raw_mail.force_encoding("ASCII-8BIT") if raw_mail.respond_to?(:force_encoding)
 
       ActiveSupport::Notifications.instrument("receive.action_mailer") do |payload|
diff --git a/app/services/incoming_emails/user_creator.rb b/app/services/incoming_emails/user_creator.rb
index f7b3f9af434..c1e465b2468 100644
--- a/app/services/incoming_emails/user_creator.rb
+++ b/app/services/incoming_emails/user_creator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #  OpenProject is an open source project management software.
 #  Copyright (C) the OpenProject GmbH
 #
diff --git a/lib/redmine/pop3.rb b/lib/redmine/pop3.rb
index f65b8413247..57b34bd4e0b 100644
--- a/lib/redmine/pop3.rb
+++ b/lib/redmine/pop3.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #-- copyright
 # OpenProject is an open source project management software.
 # Copyright (C) the OpenProject GmbH
@@ -31,7 +33,7 @@ require "net/pop"
 module Redmine
   module POP3
     class << self
-      def check(pop_options = {}, options = {})
+      def check(pop_options = {}, options = {}) # rubocop:disable Metrics/AbcSize, Metrics/PerceivedComplexity
         host = pop_options[:host] || "127.0.0.1"
         port = pop_options[:port] || "110"
         apop = (pop_options[:apop].to_s == "1")
diff --git a/spec/models/mail_handler_spec.rb b/spec/models/mail_handler_spec.rb
index ecddc7f31e1..129d455b9c4 100644
--- a/spec/models/mail_handler_spec.rb
+++ b/spec/models/mail_handler_spec.rb
@@ -30,7 +30,7 @@
 
 require "spec_helper"
 
-RSpec.describe IncomingEmails::MailHandler do
+RSpec.describe IncomingEmails::MailHandler do # rubocop:disable RSpec/SpecFilePathFormat
   # we need these run first so the anonymous and system users are created and
   # there is a default work package priority to save any work packages
   shared_let(:anno_user) { User.anonymous }
diff --git a/spec/services/incoming_emails/dispatch_service_spec.rb b/spec/services/incoming_emails/dispatch_service_spec.rb
new file mode 100644
index 00000000000..08960895555
--- /dev/null
+++ b/spec/services/incoming_emails/dispatch_service_spec.rb
@@ -0,0 +1,226 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe IncomingEmails::DispatchService do
+  let(:email) { instance_double(Mail::Message) }
+  let(:text_part) { instance_double(Mail::Part) }
+  let(:body) { instance_double(Mail::Body) }
+  let(:options) { {} }
+  let(:service) { described_class.new(email, options:) }
+
+  before do
+    allow(body).to receive_messages(decoded: "Test body".dup)
+    allow(text_part).to receive_messages(body:, charset: "UTF-8")
+    allow(email).to receive_messages(text_part: text_part, from: ["test@example.com"])
+  end
+
+  describe "#initialize" do
+    it "initializes with email and options" do
+      expect(service.email).to eq(email)
+      expect(service.options).to include(options)
+    end
+
+    it "extracts sender email from the email" do
+      expect(service.sender_email).to eq("test@example.com")
+    end
+
+    it "initializes empty logs array" do
+      expect(service.logs).to eq([])
+    end
+  end
+
+  describe ".handlers" do
+    it "returns the default handlers" do
+      expect(described_class.handlers).to include(
+        IncomingEmails::Handlers::MessageReply,
+        IncomingEmails::Handlers::WorkPackage
+      )
+    end
+  end
+
+  describe ".register_handler" do
+    let(:custom_handler) { class_double(IncomingEmails::Handlers::WorkPackage) }
+
+    after do
+      described_class.remove_handler(custom_handler)
+    end
+
+    it "adds handler to the beginning of the handlers list" do
+      described_class.register_handler(custom_handler)
+      expect(described_class.handlers).to include(custom_handler)
+    end
+  end
+
+  describe "#call!" do
+    let(:user) { build_stubbed(:user) }
+
+    before do
+      allow(service).to receive(:determine_actor)
+      allow(service).to receive_messages(ignore_mail?: false, user: user)
+      allow(service).to receive(:dispatch)
+    end
+
+    it "calls determine_actor" do
+      service.call!
+      expect(service).to have_received(:determine_actor)
+    end
+
+    it "calls dispatch when user is present" do
+      service.call!
+      expect(service).to have_received(:dispatch)
+    end
+
+    it "does not call dispatch when user is not present" do
+      allow(service).to receive(:user).and_return(nil)
+      service.call!
+      expect(service).not_to have_received(:dispatch)
+    end
+
+    it "returns early when mail should be ignored" do
+      allow(service).to receive(:ignore_mail?).and_return(true)
+      service.call!
+      expect(service).not_to have_received(:determine_actor)
+    end
+  end
+
+  describe "#ignore_mail?" do
+    it "returns false by default" do
+      allow(service).to receive_messages(mail_from_system?: false, ignored_by_header?: false, ignored_user?: false)
+
+      expect(service.send(:ignore_mail?)).to be_falsey
+    end
+  end
+
+  describe "#mail_from_system?" do
+    context "when email is from system address" do
+      before do
+        allow(Setting).to receive(:mail_from).and_return("system@example.com")
+        allow(email).to receive(:from).and_return(["system@example.com"])
+      end
+
+      it "returns true" do
+        service = described_class.new(email, options:)
+        expect(service.send(:mail_from_system?)).to be_truthy
+      end
+    end
+
+    context "when email is not from system address" do
+      before do
+        allow(Setting).to receive(:mail_from).and_return("system@example.com")
+        allow(email).to receive(:from).and_return(["user@example.com"])
+      end
+
+      it "returns false" do
+        service = described_class.new(email, options:)
+        expect(service.send(:mail_from_system?)).to be_falsey
+      end
+    end
+  end
+
+  describe "#ignored_by_header?" do
+    context "with auto-response suppress header" do
+      before do
+        allow(email).to receive(:header).and_return({ "X-Auto-Response-Suppress" => "OOF" })
+      end
+
+      it "returns true" do
+        expect(service.send(:ignored_by_header?)).to be_truthy
+      end
+    end
+
+    context "with auto-submitted header" do
+      before do
+        allow(email).to receive(:header).and_return({ "Auto-Submitted" => "auto-replied" })
+      end
+
+      it "returns true" do
+        expect(service.send(:ignored_by_header?)).to be_truthy
+      end
+    end
+
+    context "without ignored headers" do
+      before do
+        allow(email).to receive(:header).and_return({})
+      end
+
+      it "returns false" do
+        expect(service.send(:ignored_by_header?)).to be_falsey
+      end
+    end
+  end
+
+  describe "#object_reference_from_header" do
+    context "with valid reference header" do
+      before do
+        allow(email).to receive(:references).and_return(["<op.work_packages-123@example.com>"])
+      end
+
+      it "extracts object reference" do
+        reference = service.send(:object_reference_from_header)
+        expect(reference).to eq({ klass: "work_packages", id: 123 })
+      end
+    end
+
+    context "without valid reference header" do
+      before do
+        allow(email).to receive(:references).and_return([])
+      end
+
+      it "returns empty hash" do
+        reference = service.send(:object_reference_from_header)
+        expect(reference).to eq({})
+      end
+    end
+  end
+
+  describe "#instantiate_matching_handler" do
+    let(:handler_class) { class_double(IncomingEmails::Handlers::WorkPackage) }
+    let(:handler_instance) { instance_double(IncomingEmails::Handlers::WorkPackage) }
+
+    before do
+      allow(service).to receive(:object_reference_from_header).and_return({})
+      allow(described_class).to receive(:handlers).and_return([handler_class])
+      allow(handler_class).to receive_messages(handles?: true, new: handler_instance)
+    end
+
+    it "finds and instantiates matching handler" do
+      result = service.send(:instantiate_matching_handler)
+      expect(result).to eq(handler_instance)
+    end
+
+    it "returns nil when no handler matches" do
+      allow(handler_class).to receive(:handles?).and_return(false)
+      result = service.send(:instantiate_matching_handler)
+      expect(result).to be_nil
+    end
+  end
+end
diff --git a/spec/services/incoming_emails/handlers/base_spec.rb b/spec/services/incoming_emails/handlers/base_spec.rb
index 4ad4d7f2a98..c3773665b1c 100644
--- a/spec/services/incoming_emails/handlers/base_spec.rb
+++ b/spec/services/incoming_emails/handlers/base_spec.rb
@@ -55,5 +55,104 @@ RSpec.describe IncomingEmails::Handlers::Base do
         expect(subject.cleaned_up_text_body).to eq("Subject:foo\nDescription:bar")
       end
     end
+
+    context "with string delimiters" do
+      before do
+        allow(Setting).to receive_messages(mail_handler_body_delimiters: "---", mail_handler_body_delimiter_regex: "")
+      end
+
+      it "removes content after delimiter" do
+        body = "Before delimiter\n---\nAfter delimiter"
+        result = subject.send(:cleanup_body, body)
+        expect(result).to eq("Before delimiter")
+      end
+    end
+  end
+
+  describe "#get_keyword" do
+    let(:plain_text_body) { "Project: test\nStatus: Open\nSome content here".dup }
+    let(:options) { { allow_override: [:project], issue: {} } }
+
+    it "extracts keyword from body" do
+      result = subject.send(:get_keyword, :project)
+      expect(result).to eq("test")
+    end
+
+    it "returns nil for non-existent keyword" do
+      result = subject.send(:get_keyword, :nonexistent)
+      expect(result).to be_nil
+    end
+
+    it "respects allow_override option" do
+      result = subject.send(:get_keyword, :status)
+      expect(result).to be_nil # status not in allow_override
+    end
+  end
+
+  describe "#extract_keyword!" do
+    it "extracts and removes keyword from text" do
+      text = "Project: test\nStatus: Open\nSome content here".dup
+      result = subject.send(:extract_keyword!, text, :project, nil)
+      expect(result).to eq("test")
+      expect(text).not_to include("Project: test")
+    end
+
+    it "handles case insensitive matching" do
+      text = "PROJECT: test\nContent".dup
+      result = subject.send(:extract_keyword!, text, :project, nil)
+      expect(result).to eq("test")
+    end
+  end
+
+  describe "#human_attr_translations" do
+    let(:user) { build_stubbed(:user, language: "en") }
+
+    it "returns array of attribute translations" do
+      result = subject.send(:human_attr_translations, :project)
+      expect(result).to include("project", "Project")
+    end
+
+    it "includes translations for user language" do
+      allow(Setting).to receive(:default_language).and_return("en")
+      result = subject.send(:human_attr_translations, :project)
+      expect(result).to be_an(Array)
+      expect(result).not_to be_empty
+    end
+  end
+
+  describe "#ignored_filename?" do
+    before do
+      allow(Setting).to receive(:mail_handler_ignore_filenames).and_return("signature.asc\n*.tmp")
+    end
+
+    it "returns true for ignored filenames" do
+      expect(subject.send(:ignored_filename?, "signature.asc")).to be_truthy
+    end
+
+    it "returns false for non-ignored filenames" do
+      expect(subject.send(:ignored_filename?, "document.pdf")).to be_falsey
+    end
+  end
+
+  describe "#lookup_case_insensitive_key" do
+    let(:scope) { class_double(Status) }
+    let(:status) { build_stubbed(:status) }
+    let(:options) { { allow_override: [:status], issue: {} } }
+    let(:plain_text_body) { "Status: resolved".dup }
+
+    before do
+      allow(scope).to receive(:find_by).with("lower(name) = ?", "resolved").and_return(status)
+    end
+
+    it "finds record case insensitively" do
+      result = subject.send(:lookup_case_insensitive_key, scope, :status)
+      expect(result).to eq(status.id)
+    end
+
+    it "returns nil when keyword not found" do
+      allow(scope).to receive(:find_by).and_return(nil)
+      result = subject.send(:lookup_case_insensitive_key, scope, :status)
+      expect(result).to be_nil
+    end
   end
 end
diff --git a/spec/services/incoming_emails/handlers/message_reply_spec.rb b/spec/services/incoming_emails/handlers/message_reply_spec.rb
new file mode 100644
index 00000000000..43ab1a4e2e6
--- /dev/null
+++ b/spec/services/incoming_emails/handlers/message_reply_spec.rb
@@ -0,0 +1,113 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe IncomingEmails::Handlers::MessageReply do
+  let(:email) { instance_double(Mail::Message, attachments: [], subject: "Email subject") }
+  let(:user) { build_stubbed(:user) }
+  let(:reference) { {} }
+  let(:options) { {} }
+  let(:plain_text_body) { "Test body" }
+
+  subject(:handler) do
+    described_class.new(email, user:, reference:, plain_text_body:, options:)
+  end
+
+  describe ".handles?" do
+    context "with message reference" do
+      let(:reference) { { klass: "message", id: 123 } }
+
+      it "returns true" do
+        expect(described_class).to be_handles(email, reference:)
+      end
+    end
+
+    context "without message reference" do
+      let(:reference) { {} }
+
+      it "returns false" do
+        expect(described_class).not_to be_handles(email, reference:)
+      end
+    end
+
+    context "with work package reference" do
+      let(:reference) { { klass: "work_packages", id: 123 } }
+
+      it "returns false" do
+        expect(described_class).not_to be_handles(email, reference:)
+      end
+    end
+  end
+
+  describe "#process" do
+    let(:project) { create(:project) }
+    let(:forum) { create(:forum, project:) }
+    let(:message) { create(:message, forum:) }
+    let(:reference) { { klass: "messages", id: message.id } }
+
+    before do
+      allow(Message).to receive(:find_by).with(id: message.id).and_return(message)
+    end
+
+    context "when not allowed to create a message" do
+      it "raises an exception" do
+        expect { handler.process }.to raise_error(IncomingEmails::UnauthorizedAction)
+      end
+    end
+
+    context "when allowed to create a message" do
+      before do
+        mock_permissions_for(user) do |mock|
+          mock.allow_in_project(:add_messages, project:)
+        end
+      end
+
+      it "creates a reply message" do
+        call = handler.process
+        expect(call).to be_a(ServiceResult)
+
+        message = call.result
+        expect(message).to be_a(Message)
+      end
+    end
+
+    context "when parent message is not found" do
+      before do
+        allow(Message).to receive(:find_by).and_return(nil)
+      end
+
+      it "returns nil" do
+        result = handler.process
+        expect(result).to be_nil
+      end
+    end
+  end
+end
diff --git a/spec/services/incoming_emails/handlers/work_package_spec.rb b/spec/services/incoming_emails/handlers/work_package_spec.rb
new file mode 100644
index 00000000000..ec815e83fa0
--- /dev/null
+++ b/spec/services/incoming_emails/handlers/work_package_spec.rb
@@ -0,0 +1,98 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe IncomingEmails::Handlers::WorkPackage do
+  let(:email) { instance_double(Mail::Message, attachments: [], subject: "My message") }
+  let(:user) { build_stubbed(:user) }
+  let(:reference) { {} }
+  let(:options) { { issue: { project: "foobar" }, allow_override: [] } }
+  let(:plain_text_body) { "Test body".dup }
+
+  subject(:handler) do
+    described_class.new(email, user:, reference:, plain_text_body:, options:)
+  end
+
+  describe ".handles?" do
+    context "with work package reference" do
+      let(:reference) { { klass: "work_package", id: 123 } }
+
+      it "returns true" do
+        expect(described_class).to be_handles(email, reference:)
+      end
+    end
+
+    context "without work package reference" do
+      let(:reference) { {} }
+
+      it "returns true for new work package creation" do
+        expect(described_class).to be_handles(email, reference:)
+      end
+    end
+
+    context "with message reference" do
+      let(:reference) { { klass: "messages", id: 123 } }
+
+      it "returns false" do
+        expect(described_class).not_to be_handles(email, reference:)
+      end
+    end
+  end
+
+  describe "#process" do
+    let(:service_result) { ServiceResult.success(result: build_stubbed(:work_package)) }
+    let(:service_instance) { instance_double(WorkPackages::CreateService, call: service_result) }
+    let(:project) { build_stubbed(:project) }
+
+    before do
+      allow(WorkPackages::CreateService).to receive(:new).and_return(service_instance)
+      allow(Project).to receive(:find_by).with(identifier: "foobar").and_return(project)
+    end
+
+    it "creates a work package" do
+      result = handler.process
+      expect(WorkPackages::CreateService).to have_received(:new)
+      expect(result).to be_a(ServiceResult)
+    end
+
+    context "when work package creation fails" do
+      let(:errors) { ActiveModel::Errors.new(nil) }
+      let(:service_result) { ServiceResult.failure(errors:) }
+
+      it "returns the failed result" do
+        result = handler.process
+        expect(result).to be_a(ServiceResult)
+        expect(result).not_to be_success
+        expect(result.errors).to eq(errors)
+      end
+    end
+  end
+end
diff --git a/spec/services/incoming_emails/mail_handler_integration_spec.rb b/spec/services/incoming_emails/mail_handler_integration_spec.rb
new file mode 100644
index 00000000000..4b10ad44e11
--- /dev/null
+++ b/spec/services/incoming_emails/mail_handler_integration_spec.rb
@@ -0,0 +1,267 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe IncomingEmails::MailHandler, "integration", type: :service do
+  # Integration tests to ensure backwards compatibility after refactoring
+
+  shared_let(:project) { create(:valid_project, identifier: "test-project") }
+  shared_let(:user) { create(:user, mail: "test@example.com") }
+  shared_let(:priority) { create(:priority_low, is_default: true) }
+
+  describe "MailHandler.receive" do
+    let(:raw_email) do
+      <<~EMAIL
+        From: test@example.com
+        To: project@example.com
+        Subject: Test Work Package
+        Content-Type: text/plain
+
+        Project: #{project.identifier}
+
+        This is a test work package created via email.
+      EMAIL
+    end
+
+    it "still processes emails through the old API" do
+      result = described_class.receive(raw_email)
+
+      expect(result).to be_a(WorkPackage)
+      expect(result.subject).to eq("Test Work Package")
+      expect(result.project).to eq(project)
+      expect(result.author).to eq(user)
+      expect(result.description).to include("This is a test work package created via email.")
+    end
+
+    it "handles options parameter" do
+      result = described_class.receive(raw_email, { allow_override: "priority" })
+
+      expect(result).to be_a(WorkPackage)
+      expect(result.project).to eq(project)
+    end
+
+    context "with unknown user" do
+      let(:unknown_email) do
+        <<~EMAIL
+          From: unknown@example.com
+          To: project@example.com
+          Subject: Test from unknown
+          Content-Type: text/plain
+
+          Project: #{project.identifier}
+
+          This is from an unknown user.
+        EMAIL
+      end
+
+      it "handles unknown_user option, but does not skip permission checks" do
+        result = described_class.receive(unknown_email, { unknown_user: "accept" })
+        expect(result).to be_a(WorkPackage)
+        expect(result.project).to eq(project)
+
+        expect(result.errors.symbols_for(:base)).to contain_exactly(:error_unauthorized)
+      end
+    end
+
+    context "with auto-reply headers" do
+      let(:auto_reply_email) do
+        <<~EMAIL
+          From: test@example.com
+          To: project@example.com
+          Subject: Test Auto Reply
+          X-Auto-Response-Suppress: OOF
+          Content-Type: text/plain
+
+          Project: #{project.identifier}
+
+          This is an auto-reply.
+        EMAIL
+      end
+
+      it "ignores auto-reply emails" do
+        result = described_class.receive(auto_reply_email)
+
+        expect(result).to be_nil
+      end
+    end
+
+    context "with work package reference" do
+      let!(:work_package) { create(:work_package, project:) }
+      let(:options) { {} }
+      let(:reply_email) do
+        <<~EMAIL
+          From: test@example.com
+          To: project@example.com
+          Subject: Re: #{work_package.subject}
+          References: <op.work_package-#{work_package.id}@example.com>
+          Content-Type: text/plain
+
+          This is a reply to the work package.
+        EMAIL
+      end
+
+      subject { described_class.receive(reply_email, options) }
+
+      shared_examples "successful work package reply" do
+        it "processes work package replies" do
+          expect { subject }.to change(Journal, :count).by(1)
+
+          expect(subject).to be_a(Journal)
+          expect(subject.journable).to eq(work_package)
+          expect(subject.notes).to include("This is a reply to the work package.")
+        end
+      end
+
+      context "with permission" do
+        let!(:role) { create(:project_role, permissions: %i[view_work_packages add_work_package_comments]) }
+        let!(:member) { create(:member, project:, user:, roles: [role]) }
+
+        it_behaves_like "successful work package reply"
+      end
+
+      context "without permission" do
+        it "fails to process" do
+          expect { subject }.not_to change(Journal, :count)
+        end
+      end
+
+      context "without permission, but no_permission_check set" do
+        let(:options) do
+          { no_permission_check: true }
+        end
+
+        it_behaves_like "successful work package reply"
+      end
+    end
+
+    context "with message reference" do
+      let(:options) { {} }
+      let!(:message) { create(:message, forum: create(:forum, project:)) }
+      let(:message_reply_email) do
+        <<~EMAIL
+          From: test@example.com
+          To: project@example.com
+          Subject: Re: #{message.subject}
+          References: <op.message-#{message.id}@example.com>
+          Content-Type: text/plain
+
+          This is a reply to the message.
+        EMAIL
+      end
+
+      subject { described_class.receive(message_reply_email, options) }
+
+      shared_examples "successful message reply" do
+        it "processes message replies" do
+          expect { subject }.to change(Message, :count).by(1)
+
+          expect(subject).to be_a(Message)
+          expect(subject.parent).to eq(message)
+          expect(subject.content).to include("This is a reply to the message.")
+        end
+      end
+
+      context "with permission" do
+        let!(:role) { create(:project_role, permissions: [:add_messages]) }
+        let!(:member) { create(:member, project:, user:, roles: [role]) }
+
+        it_behaves_like "successful message reply"
+      end
+
+      context "without permission" do
+        it "fails to process" do
+          expect { subject }.not_to change(Message, :count)
+          expect(subject).to be_nil
+        end
+      end
+
+      context "without permission, but no_permission_check set" do
+        let(:options) do
+          { no_permission_check: true }
+        end
+
+        it_behaves_like "successful message reply"
+      end
+    end
+  end
+
+  describe "Error handling" do
+    let(:invalid_email) do
+      <<~EMAIL
+        From: test@example.com
+        To: project@example.com
+        Subject: Invalid Email
+        Content-Type: text/plain
+
+        Project: nonexistent-project
+
+        This references a nonexistent project.
+      EMAIL
+    end
+
+    it "handles errors gracefully" do
+      expect { described_class.receive(invalid_email) }.not_to raise_error
+    end
+  end
+
+  describe "Handler registration" do
+    let(:custom_handler) do
+      Class.new(IncomingEmails::Handlers::Base) do
+        def self.handles?(email, **)
+          email.subject&.include?("CUSTOM")
+        end
+
+        def process
+          ServiceResult.success(message: "Custom handler processed", result: "foo result")
+        end
+      end
+    end
+
+    let(:custom_email) do
+      <<~EMAIL
+        From: test@example.com
+        To: project@example.com
+        Subject: CUSTOM Test Email
+        Content-Type: text/plain
+
+        This should be handled by the custom handler.
+      EMAIL
+    end
+
+    it "allows custom handlers to be registered" do
+      IncomingEmails::DispatchService.register_handler(custom_handler)
+
+      result = described_class.receive(custom_email.dup)
+
+      expect(result).to eq("foo result")
+    end
+  end
+end
diff --git a/spec/models/mail_handler/user_creator_spec.rb b/spec/services/incoming_emails/user_creator_spec.rb
similarity index 98%
rename from spec/models/mail_handler/user_creator_spec.rb
rename to spec/services/incoming_emails/user_creator_spec.rb
index 51ca264b90a..bf16adac9ee 100644
--- a/spec/models/mail_handler/user_creator_spec.rb
+++ b/spec/services/incoming_emails/user_creator_spec.rb
@@ -28,7 +28,7 @@
 
 require "spec_helper"
 
-RSpec.describe MailHandler::UserCreator do
+RSpec.describe IncomingEmails::UserCreator do
   describe ".new_user_from_attributes" do
     context "with sufficient information" do
       # [address, name] => [login, firstname, lastname]